1. Field of The Invention
This invention relates to a method for manufacturing an oxygen sensor unit which is suitable for use in oxygen sensors of internal combustion engines for automotive vehicles.
2. Description of the Prior Art
For the control of an air-to-fuel ration in internal combustion engines of automotive vehicles, oxygen sensors have been usually used. A typical structure of such a sensor is, for example, shown in FIG. 4 which will be described hereinafter. An oxygen sensor unit incorporated in the oxygen sensor is, for example, of the type which comprises a solid electrolyte body, and a pair of electrodes formed on inner and outer sides of the solid electrolyte, respectively. Moreover, a protective layer is provided to cover the outer electrode therewith at the outer surface thereof so as to protect the outer electrode from a poisonous contaminant in a gas to be measured as shown in FIG. 3. This figure will also be referred to hereinafter.
The solid electrolyte of the oxygen sensor unit has been conventionally made, for example, of a sintered body of zirconia to which a stabilizer is added.
The sintered bodies of the zirconia known in the art can be broadly classified into two groups. One group includes a body made of fully stabilized zirconia which consists essentially of a cubic phase (C phase) alone. The other group includes a body made of partially stabilized zirconia which is constituted mainly of a cubic phase (C phase), a monoclinic phase (M phase) and/or a tetragonal phase (T phase) existing in mingling relation.
The fully stabilized zirconia is one which is stable over a wide temperature range of room temperature (20.degree. C.) to a high temperature of 1000.degree. C. and is unlikely to degrade as time passes. However, this type of zirconia is neither resistant to mechanical shocks such as vibrations, nor resistant to thermal shocks, thus being liable to break. Owing to this deficiency of the fully stabilized zirconia, partially stabilized zirconia sintered bodies have been usually employed as a solid electrolyte in this field of the art.
However, when partially stabilized zirconia is repeatedly subjected to heating and cooling cycles at temperatures between room temperature (20.degree. C.) and a high temperature of 1000.degree. C., phase transformation takes place, as is shown in FIG. 5, between the monoclinic phase (i.e. monoclinic zirconia in the figure) and the tetragonal phase (tetragonal zirconia in FIG. 5).
As will be apparent from FIG. 5, the phase transformation involves a great variation in volume. When using partially stabilized zirconia in an oxygen sensor unit as a solid electrolyte body and subjecting the unit to heating and cooling cycles, this solid electrolyte body may undesirably be cracked or, in the worst case, broken.
In usual practice, a protective layer is formed to protect an outside electrode which is exposed to a gas to be measured as described before. In this condition, if phase transformation accompanying the volumetric variation occurs between the M and T phases of the partially stabilized zirconia sintered body serving as the solid electrolyte, some stress is caused to occur between the solid electrolyte body and the protective layer, with the great possibility that the protective layer suffers cracks and the protective layer separates from the solid electrolyte body or the outer electrode.